1. Digestive System
Chapter 18

2. Function Obtain resources from the external environment
Water
Minerals
Nutrients (Lipids, Carbohydrates, Proteins)
Vitamins
Break down large particles into smaller ones
Transfer materials from external environment  blood  cells

3. Anatomy Fig 18.2 Digestive tract oral cavity pharynx esophagus stomach
small intestine
large intestine
anus
Fig 18.2

4. Anatomy Fig 18.2 Accessory organs salivary glands pancreas liver
gall bladder
Fig 18.2

5. Digestive Processes Motility Secretion Digestion Absorption
movement of food through the digestive system
Secretion
release of substances to enhance breakdown of food
Digestion
physical and chemical breakdown of food
Absorption
transfer of materials to internal environment

6. Motility Figs 18.4, 18.14 Muscle contractions of digestive tract
propulsion of food through GI tract
mixing of food with digestive juices (facilitates digestion and absorption)
Processes:
ingestion (taking food into mouth)
mastication (chewing)
deglutition (swallowing)
peristalsis (rhythmic movement of GI tract)
segmentation (mixing in intestine)
Figs 18.4,
18.14

7. Secretion Digestive juices Most food molecules too large to absorb
enzymes, bile salts, mucus, etc. released by exocrine glands into GI tract
Most food molecules too large to absorb
Must break bonds with enzymes (various organs)
Enzyme function aided by…
HCl (stomach)
Bile (liver)
NaHCO3 (pancreas)

8. Digestion
Physical and chemical break down nutrients into absorbable unit
Physical digestion (chewing, mixing)
Chemical digestion (enzyme catalyzed)
polysaccharides  monosaccharides
proteins  amino acids
fats  glycerol + fatty acids

9. Absorption
Transfer of digested materials across the digestive epithelium
intestinal lumen  blood or lymph

10. Gastrointestinal Tract Structure
Mucosa (lumen side)
Epithelial tissue
Submucosa
elastic connective tissue
contains lymph and blood vessels
Muscularis
smooth muscle
Serosa
outer layer of connective tissue
secretes serous fluid
Fig 18.3

11. Mouth Fig 18.2 Ingestion - bringing food into the body
tongue - taste buds detect chemical composition of food
Mastication - chewing (physical digestion)
teeth and tongue
Chemical digestion - saliva
moistens food
amylase - breaks down starch into maltose
lysozyme - antibacterial agent
Fig 18.2

12. Pharynx and Esophagus Figs 18.2, 18.4
Transport food and water to stomach, secretes mucus
deglutition (swallowing) reflex moves food to stomach
Movement of food bolus in esophagus (and rest of GI tract) via peristalsis
Figs 18.2, 18.4

13. Stomach Fig 18.5 Muscular sac-like organ
Chemical and physical digestion
forms chyme
Stores food, releases small amts. to small intestine
takes 2-6 hours for stomach to empty
inner surface lined w/ rugae
Fig 18.5

14. Stomach Mucosal Cells Figs 18.7-18.9 goblet cells – mucus
Gastric Mucosal Barrier = mucus and layer of HCO3 that protects stomach epithelium
parietal cells – HCl
kills bacteria, denatures proteins
chief cells - pepsinogen
pepsinogen activated by HCl  pepsin
pepsin breaks down proteins
Figs

15. Small Intestine Figs 18.10-18.13 12’ long tube (20’ in cadavers)
Duodenum (1’)  Jejunem (4’)  Ileum (7’)
Most chemical digestion occurs here
Most absorption occurs here
Large surface area
Plicae – folds in mucosa
Villi – fingerlike projections
Capillaries, central lacteal (absorption)
Microvilli ("brush border") on epithelium
Figs

16. Chemical Digestion: Small Intestine
brush-border enzymes
bound to epithelial cell membranes
hydrolyze disaccharides, polypeptides, etc.
e.g. enterokinase - activates trypsin (pancreatic enz.)
Fig 18.29

17. Chemical Digestion: Pancreas
Exocrine cells secrete pancreatic juice into duodenum
amylase - breaks down starch
trypsinogen
converted to trypsin by enterokinase
breaks down polypeptides
lipase - digests triglycerides
many others (proteases, nucleases, etc.)
NaHCO3 (alkaline) - neutralizes stomach acidity
Fig 18.26

18. Chemical Digestion: Liver
secretes bile
stored in gall bladder, secreted into duodenum
bile salts used for lipid absorption
bilirubin secretion
metabolic processing of absorbed materials
Nutrient-rich blood from villi flows into hepatic portal system
processed by liver prior to general circulation
Fig 18.22

19. Chemical Digestion Fig 18.22
degradation of waste, hormones, drugs, etc.
synthesizes plasma proteins
stores glycogen, fats, minerals and vitamins
Fig 18.22

20. Absorption: Carbohydrates and Proteins
digested into monosaccharides and amino acids / small peptides
transported into the epithelium by cotransport
coupled to flow of Na+ or H+ into epithelial cells
transported into blood entering hepatic portal system
Figs 18.33, 18.22

21. Absorption: Lipids Figs 18.25, 18.35, and 18.36 emulsification
lipids interact w/ bile salts
form emulsification droplets
pancreatic lipases break emulsified fat into free fatty acids and monoglycerides
absorbed by epithelium
Figs 18.25, 18.35,
and 18.36

22. Absorption: Lipids Fig 18.36 reform triglycerides in epithelial cells
combined w/ protein to form chylomicrons
chylomicrons released via exocytosis into submucosa
enter central lacteal  lymphatic system
transported to blood
Fig 18.36

23. Large Intestine Fig 18.17 colon, cecum, appendix, rectum (5' long)
small amount of salt and water absorbed here
main function is to store undigested material (feces)
30% dry weight of feces = bacteria (E. coli)
live in large intestine (produce vitamin K)
Fig 18.17